Field of the Invention
The present invention relates to a method for the production of polysaccharide fibers which, as a fiber-forming substance, comprise a mixture of cellulose and α(1→3)-glucan, as well as to the fibers made thereof and to their use.
Description of Related Art
Polysaccharides are becoming increasingly important, as they are materials that can be obtained from renewable raw materials. One of the most frequently occurring polysaccharides is cellulose. Cotton fibers, which consist almost exclusively of cellulose, are an example of the significance of polysaccharides. However, also materials obtained from other cellulosic raw materials, e.g., cellulosic synthetic fibers, are continuing to gain in importance.
The generic name “lyocell fibers” was assigned by BISFA (the International Bureau for the Standardization of Man-made Fibers) to cellulose fibers produced from solutions in an organic solvent without the formation of a derivative.
However, to date, only one method for the large-scale production of fibers of the lyocell type has gained acceptance, namely, the amine-oxide process. In this method, a tertiary amine oxide, preferably N-methylmorpholine-N-oxide (NMMO), is used as the solvent.
Tertiary amine oxides have long been known as alternative solvents for cellulose. From U.S. Pat. No. 2,179,181, it is known, for example, that tertiary amine oxides are capable of dissolving pulp without derivatization and that cellulosic shaped bodies, e.g., fibers, can be made from these solutions. U.S. Pat. No. 3,447,939 describes cyclic amine oxides as solvents for cellulose.
From numerous patent specifications and other publications, it has been known to those skilled in the art for quite some time how this method is carried out. EP 356 419 B1, for example, describes how the solution is prepared, and EP 584 318 B1 describes how such solutions of cellulose in water-containing tertiary amine oxides are spun.
The main cellulosic raw material used in the amine oxide process is pulp obtained from wood. The cellulose molecules existing in wood and also in other plant-based sources of cellulose such as cotton linters, straw, etc. form very long chains, i.e., they exhibit a high degree of polymerization. In order to obtain a cellulose spinning solution that is well suited for large-scale processing, it is necessary to specifically adjust the degree of polymerization of the cellulose molecules, which inevitably causes part of the polymer molecules to be shortened. This takes place in the usual pulp preparation methods and also in separate pretreatment steps such as bleaching, acid treatment, or irradiation by splitting the originally long cellulose molecules. In addition to the shorter chains having the desired degree of polymerization, this also creates significantly shorter fragments such as oligomers or even monomers which remain in solution after the precipitation of the spinning solution in the precipitation bath, do not contribute to the formation of the fibers, and thus are lost. The quantities of raw material lost in this process can be substantial and can affect the cost-effectiveness of the entire amine oxide process.
U.S. Pat. No. 7,000,000 describes fibers obtained by spinning a solution of polysaccharides which substantially consist of repeating hexose units linked via α(1→3)-glycosidic bonds. These polysaccharides can be produced by bringing an aqueous solution of saccharose into contact with GtfJ glucosyltransferase, isolated from Streptococcus salivarius (Simpson et al. Microbiology, vol. 41, pp 1451-1460 (1995)). As used in this context, “substantially” means that within the polysaccharide chains there may exist occasional defective locations where other bond configurations may occur. For the purposes of the present invention, these polysaccharides are referred to as “α(1→3)-glucan”.
U.S. Pat. No. 7,000,000 first discloses possibilities for the enzymatic production of α(1→3)-glucan from monosaccharides. In this way, relatively short-chained polysaccharides can be produced without the loss of monomer units, as the polymer chains are built from the monomer units. Contrary to the production of short-chained cellulose molecules, the production of α(1→3)-glucan keeps getting less expensive the shorter the polymer chains are, as in that case the required residence time in the reactors will be short.
According to U.S. Pat. No. 7,000,000, the α(1→3)-glucan is to be derivatized, preferably acetylated. Preferably, the solvent is an organic acid, an organic halogen compound, a fluorinated alcohol, or a mixture of such components. These solvents are costly and complex to regenerate.
Therefore, attempts were made to use α(1→3)-glucans instead of cellulose in an amine oxide process under commercially applied large-scale process conditions. Unfortunately, it was found that under these conditions α(1→3)-glucans could not be processed satisfactorily into fibers.